The present invention relates generally to a clutch system. In particular, the present invention relates to a torque sensing clutch.
Variable speed belt drives are commonly used with internal combustion engines, and particularly with snowmobiles and all terrain vehicles. Such belt drives permit operation from low velocities to speeds exceeding 100 mph. The belt drive typically includes a driving clutch with a shaft that is coaxial with the output shaft of the vehicle's engine. The driving or primary clutch has a stationary and a fixed sheave that together define a pulley around which a V-belt may travel. The V-belt also travels around a driven or secondary clutch pulley that transfers the engine's power to the output shaft driving the vehicle.
The effective radius of both the primary and the secondary pulley may be varied. The ratio of the primary pulley radius to the secondary pulley radius determines the ratio of engine rotational speed to output shaft rate of rotation. When the primary clutch radius is small compared to the secondary clutch, the output shaft will turn at a rate that is slower than the engine speed resulting in a relatively low vehicle speed. As the ratio of the primary and secondary clutch radii approaches 1:1, the output shaft speed is approximately equal to the engine or crank shaft speed. Then, as the primary pulley radius becomes greater than the radius of the secondary clutch pulley, an overdrive condition exists in which the output shaft is turning at a greater rate than the engine crank shaft.
Ideally, an engine will deliver power in a linear manner, and all of the available engine power is delivered to the output shaft regardless of the vehicle's speed or load. Unfortunately, this is not the case with real world engines. A typical engine will instead deliver its maximum power over a narrow range of relatively high crankshaft speeds with power falling off measurably on either side of that range. An optimum transmission would permit the engine to operate within that range regardless of the load on the engine. The engine driven primary clutch is therefore mounted to the power source and maintains the engine rpm at a value where the most power is being produced by the engine. The primary clutch also controls the engagement and disengagement of the engine from the load (the track in the case of a snowmobile) in order to start and stop vehicle movement. The secondary clutch is attached to the load (through the output shaft, gears, and track) and changes the ratio of the two clutches as the load varies. This function is performed by the torque sensing cam that can be considered the heart of the secondary clutch.
As a conventional torque sensing driven clutch upshifts to a higher ratio, the movable sheave opens the spacing between it and the fixed sheave and simultaneously rotates backwards against rotation of the fixed sheave. This backwards rotation is caused by the cam angle in the cam that is attached to the stationary sheave or the output shaft. This rotational movement on the upshift makes the clutch open to a smaller diameter, thus making a higher ratio. However, this backward rotation of the movable sheave also causes belt slippage on the movable sheave, because the sheave is trying to rotate the same direction as the stationary sheave due to the direction that the V-belt is being pulled. The only force keeping the sheaves from opening against the track load is a spring compression force and a torsional force as the clutch opens to a higher ratio. The compression force and torsional twist in the spring keeps the side load on the V-belt, which is being squeezed between the two pulleys. Again, the V-belt is slipping on the movable sheave, and the driving force is being applied to the fix sheave.
In the backshift mode, the clutch is in an open position, and the V-belt must be pushed toward the top of the pulley, thus a larger diameter, to obtain a lower ratio. The load on the track against the forward rotation makes the cam apply a closing force to the movable sheave along with the compression of the spring and the torsional unwinding of the spring. In addition, the movable sheave is being pulled rotationally forward by the V-belt. The V-belt slips rotationally on the sheaves as it moves to a larger diameter of the backshift pulley position. Again, this opposite rotation of the sheaves causes the slippage of the V-belt during the upshift and backshift function of the clutch.
Therefore, in a conventional clutch, the V-belt is working with two different pulling loads. As the movable sheave opens against the pulling force of the V-belt, the V-belt slips along the movable sheave, and all the force is transferred to the portion of the V-belt that is driving the stationary sheave. Because of this slipping, friction causes heat to build in the clutches, which makes them much less efficient and causes the clutch to operate less efficiently.